The animal carcinogenicity data, uses, exposure concentrations, and regulatory guidelines for 4,4'-methylenebis(2-chloroaniline) (101144) (MBOCA) were reviewed. It was pointed out that MBOCA was a proven animal carcinogen, but no federal regulation limited the exposure to it. It was strongly carcinogenic in rats and dogs and a weak carcinogen in mice. The target organs for tumor development differed among species. In rats, the lungs, liver, and mammae were target organs, while the bladder was most affected in dogs. Other chemicals with aromatic amine groups also have been shown to produce bladder malignancies. Certain structural similarities among them were discussed. There was no evidence of carcinogenicity in humans, but no adequate epidemiological studies had been conducted. However, since the average latency time was 20 years, it might be difficult to perform the proper studies. A prospective study was initiated by NIOSH to investigate the incidence of bladder cancer and the mortality among workers exposed to MBOCA in a Michigan chemical plant between 1968 and 1979. Almost all MBOCA was used as a curing agent for isocyanate containing polymers such as polyurethane. It was estimated that between 2,100 and 33,000 workers in the United States were exposed to MBOCA. The exposure concentrations in air samples were not extensively determined. In one study the urinary concentration of MBOCA ranged from 70 to 1,500 micrograms per liter of urine. In the U.S., MBOCA was not regulated as a carcinogen, but in several other countries it was. The authors conclude that implementation of environmental policies lag behind their formulation.